This invention more particularly relates to the recording and replication of audio and/or video information. Specifically, it deals with audio or video information recorded as a relief pattern in a suitable polymer material, the formation of a metal master from the original recording and the hot stamping or embossing of the pattern in a suitable deformable plastic medium.
A commonly-used method of making many duplicates of a sound recording is to convert the sound vibrations into corresponding surface undulations or deformations in a soft, deformable surface such as a layer of lacquer on a supporting aluminum disc, replicate the undulations in a harder metal surface, and then use the metal replica to mold sound tracks into synthetic resin discs. The sound can then be reproduced on a phonograph with the aid of a pick-up unit.
Several methods for recording optical images as relief patterns in a surface have been devised. One such method involves the use of electrostatically charged, heat deformable, photoconductive recording elements with which so-called deformation images are prepared. These can be read out by special optical techniques, such as Schlieren optics. A second method involves the recording in, e.g. a photoresist film, of phase holograms consisting of ripples in an otherwise smooth surface. A third technique employs a modulated groove in which the modulation is an analog of the electrical signal provided, for instance, to a television receiver.
A common feature in each of these three approaches is that the optical information is contained in a relief pattern formed in an otherwise smooth surface or in an unmodulated groove. Replicas of these relief patterns can be formed by techniques similar to those employed in the production of phonograph records.
Examining the usual method of making duplicate sound recordings (i.e., phonograph records) in somewhat greater detail, after the sound track has been cut into a nitrocellulose lacquer surface with a vibrating cutting stylus, the surface is prepared for deposition of nickel which is to comprise the master from which stampers are made. This preparatory treatment includes suitably sensitizing the surface and then depositing a thin layer of silver by chemical reduction so that a conductive base is formed. Nickel is then deposited electrolytically on the condutive base. Electrolytic deposition baths usually operate best at elevated temperatures, for example, 50.degree. C.
After the nickel has been deposited to suitable thickness, the assembly is separated at the interface between the silver layer and lacquer substrate. A nickel and/or chromium layer is commonly deposited on the silver. As a result, this surface is not a completely faithful reproduction of the original lacquer surface; some degradation of the original recording has been introduced. Silver, rather than nickel, is deposited initially because most electroless* nickel plating baths, which might conceivably be used to deposit metal on the lacquer surface, operate satisfactorily only at temperatures of at least 70.degree. C. Such temperatures cause undesirable physical or chemical changes in lacquers. FNT *"electroless" is a term commonly applied to autocatalytic chemical plating taking place on a suitable surface.
In accordance with the present invention, it has now been found that the silvering step can be eliminated entirely in the making of metal replications of plastic surfaces. The initial deposition of metal on the modulated plastic surface can be carried out by autocatalytic chemical plating of nickel or cobalt at or slightly above room temperature, the thickness of the initial metal deposit being dependent upon immersion time in the electroless solution. The deposits are uniform and fine-grained. Moreover, although the positive photoresists cited as examples herein are known to be attacked by alkaline media, the alkaline electroless deposition baths used in the present invention do not cause detectable degradation of the photoresist surface. As a result, a faithful reproduction of the original recording is obtained.
A prior art replication process which purported to eliminate the silvering step and substitute a nickel layer in the metal replication of plastic surfaces was described in British Pat. No. 1,078,439 published Aug. 9, 1967. This involves a homogeneous as well as a heterogeneous process rather than just a heterogeneous (surface nucleated) process, and involves making up separate solutions of a nickel salt and a reducing agent, such as sodium borohydride, stabilized with sodium hydroxide. The two solutions are mixed either during the deposition process or just prior thereto. This mixture is not stable at room temperature and nickel begins to precipitate as a dark colloidal cloud as soon as the two solutions are combined. Moreover, the plating deposit is grainy and not sufficiently adherent to a smooth surface to permit easy build up of an electrolytic metal layer.